Conventionally, hydroxyalkyl(meth)acrylate is, in general, synthesized by reacting (meth)acrylic acid and an excess amount of alkylene oxide as raw materials under elevated pressure at a temperature of from 40 to 70° C. in a reaction vessel in the presence of a catalyst or a polymerization inhibitor. Following the synthesis, the reaction mixture which has been produced in the reaction vessel is transferred to a deaerator, a distillation apparatus, by increasing the pressure of the reaction vessel, opening the upper part to be under the atmospheric pressure, or using a pump, to obtain a high-purified hydroxyalkyl(meth)acrylate.
The reaction vessel used for producing such hydroxyalkyl(meth)acrylate has a blowdown valve, which is opened when discharging contents. In general, a flush valve is used as the blowdown valve, because it prevents the formation of a liquid pool in the reaction vessel.
FIG. 3 shows an example of a cross-sectional view of a flush valve. This flush valve 20 has a valve body 22 forming a passage 21, a valve rod 23 which is inserted in the passage 21 of the valve body 22 and can move up and down, a valve head 26 which is disposed on the top of the valve rod 23, and a valve seat 25 which is disposed in the passage 21 and contacts the valve head 26 when the valve rod 23 is taken down. A blowdown aperture 24 is disposed obliquely downward at the lower part of valve body 22.
In such flush valve 20, when the valve head 26 contacts the valve seat 25, the passage is blocked to stop a flow and when the valve rod 23 moves upward and the valve head 26 is separated from the valve seat 25, the passage is opened.
However, a synthetic reaction of hydroxyalkyl(meth)acrylate may produce, as by-product, diesters derived from a reaction of hydroxyalkyl(meth)acrylate with (meth)acrylic acid or dialkylene glycol monoesters derived from a reaction of hydroxyalkyl(meth)acrylate with alkylene oxide.
The hydroxyalkyl(meth)acrylate as an objective product and the aforementioned by-product are very easily polymerized, compared with other alkyl(meth)acrylate. Accordingly, polymerization inhibitor solution, which contains polymerization inhibitor such as a phenol compound, a paraphenylenediamine compound, an amine compound, a copper dialkyldithiocarbamate, or a N-oxyl compound, is added to the reaction vessel. However, in a conventional adding method, because the polymerization inhibitor solution does not contact a cover part, an upper part, or the like of the reaction vessel, vapor of hydroxyalkyl(meth)acrylate, which contacts the cover part or the upper part, may polymerize to produce a polymer called “popcorn”.
Furthermore, an addition reaction using ethylene oxide such as a synthesis of hydroxyalkyl (meth)acrylate is common to be continuously performed without cleaning the inside of the reaction vessel after transferring the reaction mixture in order to avoid impurities and improve its productivity. When the reaction is continuously carried out over a long term, an amount of the produced polymer continues to increase gradually. A part of the polymer sometimes comes off intermittently and floats in the reaction mixture.
If the polymer floats in the reaction mixture, the polymer may get stuck to the valve head or the valve rod when opening the flush valve and discharging the reaction mixture, meaning not all of the polymer is discharged and a part of the polymer remains at the bottom of the reaction vessel. In this case, when the valve rod is taken down with the polymer remaining at the bottom and the flush valve is closed, the remaining polymer may be sandwiched between contacting surfaces of the valve head and valve seat (seal surface) and degrade sealing.
In general, when a (meth)acryl ester is produced by an esterification reaction, transesterification reaction, addition reaction, or the like, (meth)acrylic acid or its ester, and an alcohol which have low toxicity and low explosiveness are used as main raw materials and thus high sealing performance is not required. However, when using alkylene oxide, which has high toxicity and high explosiveness, in such as a production of hydroxyalkyl(meth)acrylate, high sealing performance is required. For fulfilling the requirement, in production of hydroxyalkyl(meth)acrylate, the continuous production is interrupted and maintenance for removing the remaining polymer at the bottom of the reaction vessel is performed. As a result, productivity is not sufficiently high.
A method of maintaining high sealing performance of a flush valve is discussed and, for example, Japanese Utility Model Laid-Open Application No. Hei 5-8141 (FIG. 1) suggests using a valve seat for a blowdown valve of a container bottom without leaking a mixture from a reaction vessel.
However, when polymers flow in the reaction mixture, even if a flush valve has the valve seat for a blowdown valve of a container bottom described in Utility Model Laid-Open Application No. Hei 5-8141, a small amount of the polymer remains in the reaction vessel, and thus it is not possible to sufficiently prevent polymer from being sandwiched between the seal surfaces. Therefore, high sealing performance cannot be maintained.